Chromatography: phases and calculations

In a nutshell

Chromatography is one type of analytical method that can be used to separate substances. This summary will review paper chromatography, methodology and associated calculations.

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Chromatography

Definition

An analytical method used to separate substances in a mixture.

Chromatography phases

Substances move between the mobile and stationary phases until an equilibrium is formed. Different chemicals move at different speeds between the phases, causing components in a mixture to become separated.

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Example

A chemical that spends more time in the mobile phase than stationary phase will move further.

Paper chromatography

In paper chromatography, the mobile phase is the solvent (usually ethanol or water) and the stationary phase is the chromatography paper.

The amount of time chemicals spend in each phase depends on:

• How soluble they are in the solvent
• How attracted they are to the paper

Chemicals with a high solubility and less attraction to the paper spend most time in the mobile phase, hence travel further.

Separating dyes in ink

Procedure

1. Watchglass to prevent evaporation 2. Beaker 3. Solvent to be used as the mobile phase 4. TLC plate to be used as the stationary phase 5. Compounds to be tested for 6. Baseline where compounds should be dropped on to

Calculations

An $$Rf$$ value is a standard measure that compares the separation that occurs.

​$$R_f = \frac{distance\,moved\,by\,substance}{distance\,moved\,by\,solvent}$$​

This is the ratio between the distance moved by the substance and the distance moved by the solvent (solvent front).

Different chemicals have different $$Rf$$ values in different solvents. This means these measurements can be used to help identify components in compounds.

Important points:

• You measure the distance moved by the substance from the pencil baseline to the centre of the spot. 
• $$Rf$$ values have no units and range from zero to one. 
• The further a chemical moves through a stationary phase, the larger the $$Rf$$​ value.

1. Solvent front 2. Baseline, where to start measurement 3. Distance travelled by molecule 4. Distance travelled by solvent

Example 

Calculate the $$Rf$$​ value for a dye that moves $$47\ mm$$​ in a solvent that moves $$100\ mm$$.

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​$$R_f = \frac{distance\,moved\,by\,substance}{distance\,moved\,by\,solvent}$$

$$R_f=\frac{47}{100}=0.47$$​​

The $$R_f$$ value for the dye is $$\underline{0.47}$$​.